Separation of cork from the barks of trees



March 25, 1958 E. H. COLLINS ErAL .2,828,081

SEPARATION oF co'Rx FROM THE BARxs oF TREES Filed Nov. 19, 1956 Sense/v 30 Mss# 752 El? United States SEPARATIQN "F l CORK FROM 'BARKS OF TREES ErnestHobarxt Collins, Kelso, and Edward Murray Williston, Longview, Wash., .assignors to Weyerhaeuser Tim- .ber Company, Tacoma, Wash., a corporation :of Washfington Application Novembery 19, 1956, Serial No. 623,233 7 Claims. (Cl. 241-24) This invention relates to a method for separating yfrom the barks -of trees a cork fraction of -high yield and purity cork in commercial size ,to beused;as a raw material for structural elements, 4for insulation, and as a source of chemicals.

clhe -barks of the Douglas tir, the white iir, the ponderosa fpne, .the western hemlock and .other species of trees contain substantial amounts of material called cork, because of its -resemblance to the major constituent of the barkrofthercork oak. Piecesof bark from separate trees show `a wide variation of relative amounts of cork and phloem (largely a mixture of parenchyma tissue and frequently 'bast bers or stone cells), grading from pieces consisting almost entirely of phloem and having but thin lunes `:of `pork, 'to Vthose :consisting of large masses of lcork orephelloderm with small islands of phloem existing in the cork. The relative amounts of these constituents of bark apparently are `alected by such factors as Athe species of `the ptree, age, the conditions of growth and the .position of :the bark ton the tree. The Ibark of the Douglas fir, for example, .contains from ,about 5% to about 50% cork, `.While .thatof the White fir-contains .from about 5% Nto .about 604% .corkl This .cork .component occurs in layers ranging sin thickness from the barely visible of about /g inch Lto .l1/2 inches With the ngreat fbulk occurring in Slayers ranging from about 1A; to 1/2 inch in `thickness.

Due'tofthe `method of cork formation and the presence of phloem 'tissue in substantiallquantities, and since all of lthe -bark .components :are intimately commingled and itightly Vadherent toene another, 4it is difficult to yseparate cork-of high purity quality and .high yield from the barks vof trees such as Douglas r andthe White r.

IPast-attempts to separate bark components have made use of variouscomminuting devices to reduce'the particle size ofthe'bar'k yfollowed by various methods for separat- `ing `or classifying solid material such as screening.

Mere comminution of "bark does not result in a clean cutseparation ofthe cork. Rather in the .case of Douglas 'r'bark it results in the formation of a mixture Aof some small trapezoidal cork particles, a substantial .proportion of .bast libers derived from the sclerenchyma tissue, a substantial yproportion of line-particles or powderderived from the parenchyma tissue, and a large proportionof `aggregates of cork particles and bers encrusted ywith or -bound .together with the parenchyma tissue.

:Because-"the Ibark mixture contains .cork particles, cork aggregate particles, and bark ber Vparticlesfoi overlapping fsizesvand Weights, the conventional `methods of simple screening or air separation are not successfully applicable AA'forthe separation of high purity cork in substantial yields.

Also, since a substantial proportion of 4phloem is tightly adherent to the cork aggregates .or appears as ilandsin the cork particles, screening and air separation V.techniques stilldonot produce av highpun'ty cork product 2,828,08l Patented Mar. 25, 1958 ...15e even if they are successful in separatingithefcork andcork aggregates :from the bers and bar'kupowder.

Attempts to produce .a high purity cork by arepetition of the steps for comminuting andseparatinghave `the disadvantages of drastically reducing the particle size of the cork and at the Vsame time decreasing the cork yield.

It is an object of vthe present invention to provide a method or' separating a lhigh lpurity cork fraction `from the barks ofstrees.

Another object of the present invention `is -toseparate a high .purity cork fraction in substantial Y.yields I from the barks of trees.

It is a further object to separate Aa high purity cork fraction from the barks of -trees in substantial -yields Qand in a relatively large `particle size suitable for use in structural materials, insulation products, and as a source of chemicals.

Generally stated, Ythe lpresently 1described process `cornprises reducing bark having va lmoisture content of 30% to #by Weight to the Hforni of `small pieces jhav'ing maximum `cross-sectional dimensions of less than about 1/2 inch. The bark pieces are then dried to a moisture content in the range of between about 6% to 25%. Preferably, the moisture content 4should be `adjusted to a range .of from 11.0% 4to :12%. 'The dried :bark is athen separated into a relatively large particle size cork-rich Yfraction andasmall particlesize non-.cork fraction. The cork-rich fraction is then `passed ,through a differential speed rollermill which `throughcontrolled shearing action breaks up the Vcork aggregates yand removes .fthe |tightly adhering phloem Without substantially reducing the `cork `particle size. The product rleaving therollermillis separated -to remove a large `particle fraction of substantially ypure cork. Although this cork fraction has manycommercial uses ,it is preferred `topass the cork through the roller mill a second vtime with a subsequent ,separation to further reine :the traction. fIf essentially rpure cork `'is desired the large particle fraction from the flast separation is given a light rod .milling and `further separation again with 4the retained material being substantially .pure cork of a relatively large particle size.

It -Will be apparent from .the .above that a `novel .principle of the invention resides in .the removal of phloem tissue from the cork particles by means .of ashear action which does not reduce appreciably Vvthe lparticle size, as contrasted to the pulverizing, grinding methods of previous processes, thus giving a high vyield as Well =as a substantially pure cork.

Furthermore, by minimizing .any crushing action, while `emphasizing .a shearing action, .the low apparent density and natural resilience of the cork particle is largely retained as compared to an increase and decrease respectively in these characteristics when using -the -im- `pacting means of prior methods.

It is important that the moisture content `be controlled during the passage through the roller mill and subsequent steps. In the range of between about `6% ,and about 25% by weight much `of the .moisture is contained in the cork cell particles. In part, because Aof their relatively high moisture content, the cork particles are ,resistant .to the action of a differential speed vroller mill Whereas the non-fibrous phloem `tissue is relatively .dry and brittle and particularly `susceptible vto shearing stresses. If the moisture content is reduced below about 6% by Weight the cork particles `become dessicated and hard and tend to pulverize along with the phloem -material and the overall large particle size cork yield is drastically reduced.

Control `of the moisture content in the production of cork `from the barks of trees is of still further importance aseaosl when considered from the standpoint of the properties of the cork product. When the moisture content of the bark treated is not maintained during the rolling and separating operation at a level suiciently high to insure that the cork is moist, the cork cells, which are originally turgid, collapse into hard, horny particles from which the resiliency customarily associated with cork is almost completely absent. Since this valuable property may be restored to the dried cork cells only with great diculty, if at all, over-drying results in destroying the usefulness of the cork product for many applications.

As has been indicated above, the method of this invention may be applied to the bark of any tree containing substantial'amounts of cork. It also may be applied to cork rich fractions of bark. Because of their high content of cork and availability, it is particularly applicable to the recovery of cork from the barks of the Douglas iir, the white r and other commercial coniferous species.

rl`he particular objects and advantages of the invention are described in more detail in the following example and illustrated by reference to the single figure of the drawing which is a flow chart showing the passage of bark through a series of typical processing steps.

Procedure Two separate samples of Douglas fir bark Vwere selected having an original cork content of 44.7% and 24.9% by weight and designated A and B, respectively, in the table following. The samples were processed according to the method as diagrammed by the ow chart of the gure. The whole bark was comminuted by passing through a grinder, specically a Jacobsen pulverizer, driven by a 3X1 H. P. motor at a no-load speed of 4300 R. P. M. and having 1A in. screen. After passing through the grinder the moisture content of the bark was 35-40%. The small pieces of bark were dried to a moisture content of l-l2% in a non-circulating oven at approximately 180 F. and screened on a Denver- Dillon vibrating screen set with a screen inclination of about to the horizontal. The plus 30 mesh fraction is retained in the process and passed through a rollerV mill having machined steel rolls, l2 in length and 5.8" in diameter, mounted in a horizontal plane with a static roll nip clearance set at 0.007 inch. The rolls are geared for a 1 to 1.3 differential speed. In these experiments the actual speeds of the rollers were 20 and 26 R. P. M. The bark fraction was fed directly into the nip, along the length of the rolls and doctor blades were used to prevent the bark material from adhering to the rolls and recycling through the nip. The roller mill Vis water cooled to prevent sheeting caused by exudation of waxes from the cork particles. The bark is again separated by passing over the vibrating screen and the plus mesh fraction is passed again through the roller mill. The vibrating screen set is used to separate the bark material and the plus 30 mesh fraction is further rened in a rod mill havingra cylindrical cavity, 15" long and 61/2" in diameter, rotating at R. P. M. This was a batchwise operation using 300 grams of bark material and approximately 29,000 grams of steel rods consisting of 20 by l31/2) rods and 20 (1%5" by 13H/2) rods. The bark material was screened and a plus 28 mesh fraction became the nal cork product.

The screen sizes are based on the Tyler standard screen scale with a 28 mesh screen having an opening of .0232 inch which for many commercial uses of cork is a minimum particle size.

The following table shows the cork yield and purity of product for the same particle size range after each step of the process. Byway of comparison, yield and purity data are included under C for a typical analysis of a `product resulting from a method wherein a roller mill is omitted and all reiining is done in a hall mill after the initial comminuting and screening.

Percent of Starting Percent of Cork Cork Material lu'Product Retained Retained A B C A B O Original 100 100 100 44. 7 24. 9v 34.6 Comminuted and Screened-.. 96 91.4 51.8 30.1 Sheared in Roller Mill and Screened 89.7 78. 3 64. 7 48. 3 2nd Roller Mill and Screened.. 85. 4 70. 5 77. 5 64. 0 Rolled and beat in Rod Mill and Screened 76. 2 40. 9 88.2 89. 7 Ball Mill, no Roller Mill 15.0 65.0 Ball Mill, no Roller Mill A89.0

(CorkP content analyses were made by a technique which consists of immersing'the sample to be tested in a solution of carbon tetrachloride and benzene having a specilic gravity of 1.38. The cork is determined as that portion of the bark material which floats on the solution.) I

lt is readily apparent by reference to the above table that for a product of similar particle size (+28 mesh) and similar cork content (approximately 65%) the use of a dilerential speed roller mill provides a means of increasinfy the cork yield approximately 4.5 to 6 times that of previousV methods depending on the cork content of the starting bark material.

The value of the method of this invention assumes even greater importance when we observe from the above table that for a similar particle size of a bark product of approximately 90% cork content the yield by this method is in the range from about 6 to 10 times that of a pulverizing, impacting method. i

Although not wishing to be bound by any theory, it is believed that the .i ethod of this invention recognizes the inherent difierences in the elastic and friable characteristics of the various components of bark under controlled moisture contents and, at the same time, that the nature of the bond between them is such that the application of a shear rather than an impact stress provides the most efcient and eective means of rupturing such bond.

No limitation is intended by reference to a particular` type of apparatus for practicing the method of the invention, the same being limited only by the scope of the appended claims. In particular, applicants do not wish to be limited to the specilic apparatus referred to for performing Vthe comminuting7 grinding, and shearing steps, nor do applicants wish to be limited to separation by screening or to specic sizes of screen openings. ,References in the specification to particular apparatus and screen openings are made by way of illustration only, to describe a preferred manner of carrying out the steps of the method in a continuous process best suited fora production plant. Otherrapparatus for grinding and cornminuting, and for applying shearing stresses may be devised'by those skilled in the art and some variance may be made in the screening apparatus. Alternative means of separation, such as a winnower or a centrifuge, may be used by properly coordinating the variables mentionedin the specification. All such modifications and variations are included in the invention.

Having now described our invention and in what manner the same may be used, what we claim as new and desire to protect =by Letters Patent is: 1. The method of treating Douglas r bark and mixtures of components thereof for the production of a substantially pure cork fraction which comprises' comminuting the bark having a moisture content of from 30 to by weight to produce small particles having maximum cross-sectional dimensions of less than 1/z inch; drying the bark particles to a moisture content of from 6 to 25%; separating the bark particles into a vlargeparticle size cork-rich fraction and -a small particle size non-cork fraction; shearing the particles ofthe corklricrh fraction 4to break up the cork aggregates and remove the phloem; separating a second large particle fraction of cork; shearing the said second cork fraction to further refine it; separating a t-hird large particle fraction of cork; lightly rolling and beating the said third cork fraction; and separating a nal large particle fraction of essentially pure cork.

2. The method of claim l wherein the shearing steps are performed by passing the cork fractions through a dilerential speed roller mill.

3. The method of claim l wherein the rolling and beating step is performed lby passing the cork fraction through a rod mill.

4. The method of treating Douglas fir bark and mixtures of components thereof for the production of a substantially pure cork fraction which comprises comminuting the bark having a moisture content of from 30 to 80% by weight to produce small particles having maximum cross-sectional dimensions of less than 1/2 inch; drying the bark particles to a moisture content of from 6 to 25%; separating the bark particles into a large particle size cork-rich fraction and a small particle size non-cork fraction; shearing the particles of the cork-rich fraction to break up the cork aggregates and remove the phloem; and separating a second large particle fraction of cork.

5. The method of treating Douglas lr bark and mixtures of components thereof for the production of a substantially pure cork fraction which comprises comminuting the bark having a moisture content of from 30 to 80% by weight to produce small particles having maximum cross-sectional dimensions of less than 1/2 inch; drying the bark particles to a moisture content of from 6 to 25%; separating the 'bark particles into a large particle size cork-rich fraction and a small particle size non-cork fraction; shearing the particles of the cork-rich fraction to break up the cork aggregates and remove the phloem; separating a second large particle fraction of cork; lightly rolling and beating the said second cork fraction; and separating a nal large particle fraction of essentially pure cork.

6. The method of treating Douglas r bark and mixtures of components thereof for the production of a substantially pure cork fraction which comprises comminuting the bark having a moisture content of from 30 to by weight to produce small particles having maximum cross-sectional dimensions of less than l/z inch; drying the bark particles to a moisture content of from 6 to 25%; separating the bark particles into a large particle size cork-rich fraction and a small particle size noncork fraction; shearing the particles of the cork-rich fraction to break up the cork aggregates and remove the phloem; separating a second large particle fraction of cork; shearing the said second cork fraction to further refine it; and separating a third large particle fraction of cork.

7. The method of treating Douglas tir bark and mixtures of components thereof for the production of a substantially pure cork fraction which comprises comminuting the bark having a moisture content of from 30 to 80% by weight to produce small particles having maximum cross-sectional dimensions of less than l/ inch; drying the bark particles to a moisture -content of from 10 to 12%; separating the bark particles iuto a large particle size cork-rich fraction and a small particle size non-cork fraction; shearing the particles of the cork-rich fraction to break up the cork aggregates and remove the phloem; separating a second large particle fraction of cork; shearing the said second cork fraction to further refine it; separating a third large particle fraction of cork; lightly rolling and beating the said third cork fraction; and separating a final large particle fraction of essentially pure cork.

References Cited in the tile of this patent UNITED STATES PATENTS Grondal et al Feb. 3, 1953 

